Chromosome analyses were performed an hone marrow and peripheral blood leucocyte. of 117 patients with acute and chronic leukemias and myelodysplastic, myelopralifemtive and hypereosinophilic syndromes, diagnosed in a Costa Rican hospital from May 1990 to July 1992. Cytogenetic diagnosis was achieved in 69.5% of the 131 samples, the haryotype was normal in half of them. The most common chromosomal defect was the Philadelphia trenslocation. found in 80.5% of the patients with chronic myelocitic leukemia as referral diagnosis and in some other cases. Other primary and secondary chromosomal abnormalities were less frequent. The Icaryotype analysis proved useful in clinical evaluation and management.